1. Field of the Invention
The invention pertains generally to lithographic processes for producing devices such as, e.g., semiconductor devices and, more particularly, to a method for monitoring the etch depth and each end point of a resist undergoing etching (development).
2. Art Background
Lithographic processes play an important role in the manufacture of devices such as, e.g., semiconductor devices. During the manufacture of these devices lithographic processes are used to pattern substrates, such as, e.g., silicon wafers or processed silicon wafers which are, for example, wholly or partially covered by metal, silicon dioxide or polysilicon. Typically, a substrate is patterned by coating the substrate with an energy-sensitive material called a resist. Selected portions of the resist are exposed to a form of energy which induces a change in the solubility of the exposed portions in relation to a given developing agent or etchant. The more soluble portions of the resist are then removed by etching (developing) the resist with a wet chemical etchant or by employing a dry etching process, e.g., plasma etching or reactive ion etching. The resulting pattern defined in the resist is then transferred into the underlying substrate by, for example, etching or metallizing the substrate through the patterned resist.
An important consideration in the abovedescribed patterning procedure is the determination of the etch end point of the resist, i.e., the point in time when the resist has been etched through its thickness, and thus the interface between the resist and the underlying substrate has been reached. If, for example, the resist is being etched (developed) with a wet etchant which etches the resist both vertically (through its thickness) and laterally, then overetching the resist (subjecting the resist to the etchant for a longer period of time than is needed to etch through the thickness of the resist) will often result in excessive lateral etching of the resist. Such excessive lateral etching results in a loss of linewidth control during pattern transfer into the substrate. On the other hand, if the resist is being dry etched, e.g., reactive ion etched, then overetching will often result in the sputtering of the substrate, and the deposition of sputtered substrate material onto the walls of the patterned resist. Consequently, there will also be a loss of linewidth control during pattern transfer into the underlying substrate. Such losses of linewidth control are particularly significant if pattern feature sizes are a few micrometers or smaller.
Several different techniques have been used to monitor resist etch end point. Included among these are mass-spectrometric and optical interferometric techniques. In the former technique, mass-spectrometric analysis of, for example, an etching plasma is employed to determine resist etch end point. That is, for example, once a reaction product typical of the substrate is detected in the mass-spectrometric analysis of the plasma, then the interface between the resist and the substrate is assumed to have been reached, and etching is discontinued. While this technique is useful, the time resolution inherent in the analysis of reaction products, and thus the accuracy in the determination of the etch end point, is limited by the etch rate of the substrate and by the diffusion times of substrate reaction products to a detector. As a consequence, the determination of etch end point can be in error by up to several minutes. At typical plasma etch rates (of about 500 Angstroms per minute), such errors in the end point determination correspond to at least several hundred Angstroms, and often several thousand Angstroms, of the resist film thickness (typically only 1-2 .mu.m thick), which is a very significant fraction of the resist thickness, and thus a very significant error in the determination of etch end point.
With typical optical interferometric techniques, light is shined on the resist undergoing etching, and a portion of the reflected light is detected, and the intensity of the detected light is recorded. As is known, the intensity of the reflected light oscillates periodically with time (as resist thickness is reduced) because of successive constructive and destructive interferences between light rays reflected from the bottom of the grooves being etched into the resist and light rays reflected from the underlying interface between the resist and the substrate. Etch end point is generally detected by looking for sharp changes in slope, or sharp changes in the oscillation frequency, of the detected signal. But the slope of the output signal does not always change abruptly at the end point, particularly if the substrate has optical properties similar to those of the resist, i.e., if the index of refraction of the substrate is approximately equal to that of the resist. In this case, the etch end point may also be in error by an equivalent resist thickness of at least several hundred Angstroms, and often as much as several thousand Angstroms.
Thus, those engaged in the development of lithography have long sought, thus far without success, a technique for more accurately determining the etch end point of resists.